A. Y. Takeuchi

Magnetic properties of NiMn-based Heusler alloys influenced by Fe atoms replacing Mn

The influence of the Mn substitution by Fe atoms on the magnetocaloric and magnetic properties of the martensitic Ni50Mn36Sn14 Heusler-type compound has been investigated using magnetization measurements. The insertion of Fe atoms reduces the Mn-Mn AF interactions resulting in (i) a systematic decrease in the martensitic transition temperature, down to its disappearance at 15 at. % of Fe, (ii) an enhancement of the saturation magnetization, and (iii) a monotonic increase in the L21-type phase Curie temperature. The Fe substitution also induces metamagnetic transition from an incipient AF to a noncollinear spin configuration for applied magnetic fields higher than 3 T in the case of 3 and 7 at. % Fe substitutional. The exchange-bias effect is only found in compounds with a well-defined martensitic phase transition (Fe content lower than 10 at. %). The maximum of the inverse magnetic entropy change, for a field variation of 5 T, is about +12 J kg−1 K−1 and it is nearly constant for Fe content up to 7 at. %....

Magnetocaloric properties of (La, RE)Fe11.4Si1.6 compounds (RE=Y,Gd)

The mechanosynthesis process has been applied in the LaFe11.4Si1.6 compound to reduce the undesirable segregated rich-Fe phases that impair its application as a solid magnetic refrigerant. The influence of La substitution (5 at. %) by Y or Gd atoms on the magnetic and magnetocaloric properties has been also studied. Y- and Gd-substituted compounds have a magnetic ordering temperature higher than the pure La compound. While the Y-substituted compound keeps a first-order-like magnetic transition feature, the Gd-substituted one seems to suppress it. The maximum value of the magnetic entropy change of the Y compound is roughly the same as the La compound (−18 J∕kg K) but with a magnetic entropy change peak significantly broader. For the Gd-compound case a drastic reduction of the magnetic entropy change (−7 J∕kg K) is found.

Influence of grain refinement and induced crystal defects on the magnetic properties of Ni50Mn36Sn14 Heusler alloy

The effect of high energy ball milling on the structural, magnetic and magnetocaloric properties of Ni50Mn36Sn14 Heusler-type alloy has been studied. X-ray diffraction results have revealed a reduction in the crystalline grain size concomitantly with defect inclusions in the crystalline lattice, favouring a chemical disorder effect that transforms the L21–B2-type disordered structure to a simple cubic B2-type structure for increasing milling time. From magnetometry and Mossbauer spectroscopy results, a decrease in the ferromagnetic exchange interaction contribution, an enhancement of the effective exchange bias field and a significant reduction in the magnetic entropy change of the milled alloy are observed.

La(Fe1−xCox)11.44Al1.56: A composite system for Ericsson-cycle-based magnetic refrigerators

The La(Fe1−xCox)11.44Al1.56 system, with x values of 0, 0.04, 0.08, and 0.12, was investigated for its magnetocaloric potential. For selected values of Co doping, it was possible to cover a wide magnetic ordering temperature range, from 200 up to 370K, and reduce substantially the metamagnetic critical fields. At applied magnetic fields up to 5T, the maximum magnetic entropy change approaches a nearly constant value of about 5J∕kgK for Co-doped compounds, with x varying from 0.04 to 0.12. The magnetic entropy change of a prototype composite was calculated in order to obtain a constant value in a wide temperature span. The results indicate that this material can be a good candidate for magnetic refrigeration using the Ericsson cycle.

Ferromagnetic properties of fcc Gd thin films

Magnetic properties of sputtered Gd thin films grown on Si (100) substrates kept at two different temperatures were investigated using X-ray diffraction, ac magnetic susceptibility, and dc magnetization measurements. The obtained Gd thin films have a mixture of hcp and fcc structures, but with their fractions depending on the substrate temperature TS and film thickness x. Gd fcc samples were obtained when TS = 763 K and x = 10 nm, while the hcp structure was stabilized for lower TS (300 K) and thicker film (20 nm). The fcc structure is formed on the Ta buffer layer, while the hcp phase grows on the fcc Gd layer as a consequence of the lattice relaxation process. Spin reorientation phenomenon, commonly found in bulk Gd species, was also observed in the hcp Gd thin film. This phenomenon is assumed to cause the magnetization anomalous increase observed below 50 K in stressed Gd films. Magnetic properties of fcc Gd thin films are: Curie temperature above 300 K, saturation magnetization value of about 175 emu/cm3, and coercive field of about 100 Oe at 300 K; features that allow us to classify Gd thin films, with fcc structure, as a soft ferromagnetic material.

Enhancement of magnetocaloric properties near room temperature in Ga-doped Ni50Mn34.5In15.5 Heusler-type alloy

A martensitic Ni50Mn34.5In15.5 Heusler-type alloy doped with Ga was studied by x-ray diffractometry and magnetization measurements. Ga-doping does not affect the austenitic phase transition but shifts the martensitic phase transformation towards room temperature, producing an enhancement of the magnetic entropy change (ΔSM) in that temperature region. Large ΔSM-values in the Ga-doped samples are attained for an applied field of 30 kOe as opposed to the field of 50 kOe commonly found for the un-doped cases. These effects (enhancement of ΔSM-values, shift to temperatures close to 300 K, and large ΔSM-values at lower applied fields) make the Ga-doped Ni50Mn34.5In15.5 Heusler-type alloys good candidates for technological applications as a solid refrigerant.

Influence of chemical pressure in Sn-substituted Ni2MnGa Heusler alloy: Experimental and theoretical studies

The influence of the chemical pressure on the magneto-structural properties of the Ni2MnGa Heusler alloy after a gradual substitution of Ga by Sn atoms was theoretical and experimentally studied in this work. Our data clearly show that an expansion of the L21-cell volume due Sn-substitution causes a diminution of the internal structural stress and favors the austenitic state in low temperatures where martensitic phase should prevail. It is also shown that the total magnetization reduces with increasing Sn-content, a behavior explained by a reduction of the Ni-magnetic moment, since an increase of Mn-magnetic moment was theoretically calculated. The Sn-substitution effect in the Ni2MnGa compound is similar to that found in experiments performed under high applied magnetic fields, which means that in both cases there is an increase of the L21 cell-volume favoring the austenitic state in low temperatures. Magnetization values in martensitic state of the pure Ga-compound systematically reduce after consecutive M(T) thermal cycle recorded at 5 mT; an effect not yet reported within our knowledge and attributed here to modifications in local magnetic anisotropies during the field cycles.

The role of the (111) texture on the exchange bias and interlayer coupling effects observed in sputtered NiFe/IrMn/Co trilayers

Magnetic properties of sputtered NiFe/IrMn/Co trilayers grown on different seed layers (Cu or Ta) deposited on Si (100) substrates were investigated by magnetometry and ferromagnetic resonance measurements. Exchange bias effect and magnetic spring behavior have been studied by changing the IrMn thickness. As shown by X-ray diffraction, Ta and Cu seed layers provoke different degrees of (111) fcc-texture that directly affect the exchange bias and indirectly modify the exchange spring coupling behavior. Increasing the IrMn thickness, it was observed that the coupling angle between the Co and NiFe ferromagnetic layers increases for the Cu seed system, but it reduces for the Ta case. The results were explained considering (i) different anisotropies of the Co and IrMn layers induced by the different degree of the (111) texture and (ii) the distinct exchange bias set at the NiFe/IrMn and IrMn/Co interfaces in both systems. The NiFe and Co interlayer coupling angle is strongly correlated with both exchange bias ...

The effect of Co doping on the magnetic, hyperfine and transport properties of the metamagnetic LaFe11.44Al1.56 intermetallic compound

The influence of Co doping on the hyperfine, structural, magnetic and transport properties of La(Fe1−xCox)11.44Al1.56 (x = 0, 0.04, 0.08 and 0.12) compounds was investigated by Mossbauer spectroscopy, x-ray diffraction, magnetization and electric resistivity measurements. Stabilization of the NaZn13 structure by Al atoms was also investigated in Co undoped samples and a relatively large amount of α -Fe segregated phase was observed for Al content lower than 0.10, which indicate a solubility limit for Al substitution. The Mossbauer spectra indicate that Fe atoms see a chemically disordered environment, which is related to the presence of substitutional Al and/or Co atoms. The electric field gradient value at the Fe sites of the compounds is not affected by the Al substitution, with a quadrupole splitting value of 0.44 mm s−1. The compound magnetic ordering temperature varies from 195 to 360 K and can be set according to the Co content. An enhancement of the Fe effective magnetic moment with increasing Co doping has also been observed. The applied magnetic field induces antiferromagnetic (AFM) to ferromagnetic (FM) phase transition, with a field value depending on the Co concentration, and is about 5 T for x = 0 and about 0.5 T for x = 0.04. A second magnetic phase transition occurs on cooling to low temperatures from an AFM to a spin-glass-like (SGL) state for x = 0. A possibly reentrant SGL state may occur from the FM state for x = 0.04. For x > 0.04, the compounds behave as FM for measuring field of 0.04 T.

A fast and simple method for template-free preparation of α-Fe/a-Fe2B nanosheets

A simple, room temperature reduction–hydrolysis of FeCl3 in water mediated by glycerol serving as an intermediate for controlling the concentration of Fe3+ ions in solution is an efficient and practical approach for preparing α-Fe/amorphous-Fe2B nanosheets without using a template.